Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sat May 18 2024
Day 1
Valid 16Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...16Z Update...

Maintained continuity through a majority of the previously covered
areas within the SLGT and MRGL risk areas. The prospects for flash
flooding remain highest over the Southeast U.S with convergence
along the cold front slowly progressing eastward through the
period. The primary period will be through the 00z hour before the
threat wanes as the best ascent moves away from the region and we
see more isolated to scattered rainfall coverage with less
prominence in the hourly rates. Further north over AL, a remnant
MCV will move east-southeast into northern MS/AL by this afternoon
providing a focal point for convective initiation during peak
instability. The PWAT anomalies are fairly tame for a higher end
threat, but some isolated spots could see 2-3" over the course of
the afternoon with a risk of flash flooding while cells move
overhead. Given the probabilities for 1"/hr running between 40-60%
and 2"/hr around 10-15% at peak, the threat is certainly capped for
any prospects for widespread flooding, so the MRGl risk in place
will suffice for the setup. Across NC, antecedent conditions favor
a better opportunity for flash flooding concerns with streamflow
anomalies relatively high after the recent active pattern across
the region. A backdoor front will progress to the south during the
afternoon and evening with a convergent low-level ascent pattern
developing from western NC through east-central portions of the
state. Instability and PWAT anomalies are generally modest with
scattered convection likely to form within the next few hours
across the terrain west of the Triad. Some isolated totals will
likely hit the 2-3" marker with perhaps a max around 4" based on
the latest HREFpmm and higher-end deterministic, so the threat is
certainly within reason, but the isolated nature of the potential
leans this more on the cusp of a true SLGT risk and a MRGL.

Considering the downward trend of some of the probability fields
from the previous iteration of the ensemble, as well as
collaboration with the affected WFO's, decided to maintain
continuity to allow for consistent messaging and the threat being
truly isolated. There is a potential for a short term upgrade, but
not at this time.

Kleebauer


...Previous Discussion...

...Southern Alabama/Georgia into the Florida Panhandle...

Mid-level trough/weak closed low over the Lower MS Valley/Mid-
South this morning will continue eastward across southern TN today
and into the southern Appalachians by early Sunday. On its
southeast side, a stream of mid- level impulses will move across
the FL Panhandle and southern AL/GA today ahead of a cold front
that will finally clear through the area by tomorrow. SW flow out
of the Gulf will maintain relatively high precipitable water values
(~1.75" or +1.5-2 sigma) into the region ahead of the trough and
surface front, with more impressive 850-700mb moisture flux
anomalies into the region (+3 to +4 sigma). With ongoing convection
over southern LA moving eastward, expect this to continue past 12Z
along the I-10 corridor from southern MS eastward through early
afternoon. Thereafter, models suggest a loss of mid-level vorticity
dynamics to sustain heavier rainfall but this may be underdone
given the approach of the cold front with still sufficient moisture
available. Slight Risk outline was generally constrained to the
higher probabilities of >2" QPF in the 12-18Z period this morning.
Broader Marginal Risk covers much of the rest of the Southeast into
the Carolinas where isolated heavier rain rates are possible.
Trimmed away the northern/northeastern side of the Marginal Risk
where instability is low (or non-existent) on easterly flow.

Fracasso